Complex sentences for IELTS Writing and Speaking: types and errors
Quick answer
A complex sentence has a main clause and at least one subordinate clause, joined by a word such as although, because, if, when or whereas. IELTS looks for both kinds of sentence, with the complex ones mostly under control. Length is not the test: a long string of "and" clauses is still a compound sentence. Choose the subordinator for its job, punctuate it correctly, and never pair "although" with "but".
IELTS Mentor AI marks Speaking and Writing answers on the four criteria, Grammatical Range and Accuracy among them, and explains each mistake it finds.
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A complex sentence is the structure many candidates mean when they say they want to “use better grammar”. It is also a structure that often goes wrong, because each extra clause adds another place for an error. This guide defines it plainly, sorts the subordinators by the job they do, and shows the faults that hold bands down. For the criterion as a whole, start with the pillar guide to IELTS grammar.
Key takeaways
- A complex sentence has a main clause and a subordinate clause. The subordinate one cannot stand alone.
- Choose the subordinator for its job: contrast, cause, condition, time or concession.
- Length is not complexity. A long sentence joined by “and” is not a complex sentence.
- Never use “although” and “but” together in one sentence.
- IELTS publishes no list of structures and no number of complex sentences to write.
What a complex sentence is
A clause has a subject and a verb. A main clause makes sense alone: “Prices rose.” A subordinate clause starts with a word that makes it depend on something else: “although demand fell”. It leaves the reader waiting for the main clause. Join the two and you have a complex sentence: “Prices rose although demand fell.”
A simple sentence has one clause. A compound sentence joins two main clauses with “and”, “but” or “so”. A complex sentence has at least one subordinate clause. The categories matter because the IELTS descriptors expect both kinds in the middle bands, and at the higher bands a variety of complex sentences, not only one kind.
What the bands say
IELTS.org lists Grammatical Range and Accuracy as one of the four criteria, each weighted equally, in both Speaking and Writing. This site’s Writing band descriptors explorer paraphrases the public descriptors:
- At band 5, longer sentences are attempted and fail more often than short ones do.
- At band 6, short and longer sentences are mixed through the answer. Mistakes remain in the longer ones, but they rarely stop the reader understanding.
- At band 7, several kinds of complex sentence appear, not a single pattern repeated, and many sentences have nothing wrong in them.
The difference between a band 6 script and a band 7 script is therefore not the presence of complex sentences. It is their variety and their cleanliness. The explorer lists two matching traps: one rehearsed complex opener per paragraph, and complex structures saved for the introduction and the conclusion.
IELTS publishes no list of structures and no number of complex sentences. Never write one because you believe a band depends on it.
Subordinators by job
Pick the word for the relationship between your two ideas, then check its grammar.
| Job | Subordinators | Invented example |
|---|---|---|
| Contrast | whereas, while | Cars dominate in the suburbs, whereas cycling is common in the centre. |
| Concession | although, though, even though | Although the fare rose, passenger numbers stayed the same. |
| Cause | because, since, as | The service was cut because too few people used it. |
| Condition | if, unless, provided that, as long as | Congestion will fall if the charge is kept. |
| Time | when, while, before, after, once, until | Once the line opened, journey times dropped. |
A few notes on these.
Contrast and concession are different jobs. “Whereas” sets two facts side by side. “Although” admits a point and then goes against it. Writing “whereas” for a concession reads oddly.
“While” has two meanings. It can mark time (“while the bridge was closed”) or contrast (“while rail grew, road fell”). If your sentence could read either way, choose a clearer word.
“Since” and “as” have two meanings too. Either can mean time or cause. In an argument, “because” is the safest choice.
“Despite” and “in spite of” are not subordinators. They are prepositions and need a noun or an -ing form: “Despite the rise in fares”, not “Despite fares rose”.
Punctuation
When the subordinate clause comes first, put a comma after it: “Although the fare rose, passenger numbers stayed the same.” When it comes second, you normally need no comma: “Passenger numbers stayed the same although the fare rose.” Writing descriptors also mention punctuation, so this is part of the marks.
Length is not complexity
A sentence can be long and simple. “The city grew and the roads filled and the buses slowed and people complained” has four clauses and none is subordinate. It is a string, and it is the pattern the band 5 explorer cell describes as one enormous sentence held together with “and”.
A sentence can also be short and complex. “If fares fall, more people ride.” Six words, one subordinate clause, nothing to go wrong.
The goal is a sentence whose length the idea earns. If a long sentence needs rereading, split it. If three short sentences repeat the same subject, join two of them.
The common errors
“Although… but…”
- Wrong: “Although the fare rose, but passenger numbers stayed the same.”
- Right: “Although the fare rose, passenger numbers stayed the same.”
- Also right: “The fare rose, but passenger numbers stayed the same.”
Both words mark contrast, so one is enough. The same applies to “because… so”, “despite… but” and “though… yet”.
The run-on sentence
Two main clauses joined by a comma alone make a comma splice, the usual form.
- Wrong: “The fare rose, passenger numbers stayed the same.”
- Fix with a full stop: “The fare rose. Passenger numbers stayed the same.”
- Fix with a conjunction: “The fare rose, but passenger numbers stayed the same.”
- Fix with a subordinator: “Although the fare rose, passenger numbers stayed the same.”
The explorer lists comma splices at band 6 as a trap that survives every check because it looks like a natural pause. Read your punctuation by structure, not by breath.
The fragment
A subordinate clause on its own is not a sentence.
- Wrong: “Passenger numbers stayed the same. Although the fare rose.”
- Right: “Passenger numbers stayed the same although the fare rose.”
Fragments are easy to create when you add an afterthought and put a full stop in front of it. In Speaking the equivalent is a clause you start and never finish, which costs fluency as well as grammar.
A stack of subordinators
“Although, because, when and if” in one sentence is rarely clear. If you reach three subordinate clauses, ask whether two sentences would be kinder to the reader.
Using them in each skill
In Task 2, subordinators carry the logic of an argument: a concession before a position (“Although cars are convenient, …”), a condition for a solution (“If the charge is spent on rail, …”), a reason for a claim. They do work that linking words alone cannot, which the guide to linking words and coherence covers from the cohesion side. The essay structure post shows where each kind of sentence belongs in a paragraph.
In Task 1, “whereas” and “while” suit comparison, and “after” and “when” suit sequence. Keep them for places where they save space.
In Speaking, short complex sentences are the natural unit: “I stayed in because it was raining”, “if I had more time, I would”. The aim is a sentence you can finish at speed.
For a relative clause, the other main complex structure, see the guide to relative clauses. For faults outside this criterion, see common Writing mistakes.
A check you can run
Take one of your own paragraphs and do four things. Underline every subordinate clause. Count how many different subordinators you used. Check each comma against the rules above. Then read the first word of each sentence aloud: if most are the same subject, you have a pattern to vary.
Frequently asked questions
How many complex sentences do I need in an IELTS essay?
IELTS publishes no number and no list of structures. The descriptors for the middle bands expect both kinds throughout the answer, and the upper bands a wider variety of complex sentences. Count the kinds you use, not only how many sentences carry a subordinate clause.
Can I start an IELTS sentence with "because"?
Yes, as long as the clause is joined to a main clause: "Because the road was closed, the buses were late." The error is a fragment, where the "because" clause stands alone between full stops with no main clause. When you put the subordinate clause first, finish it with a comma.
Is "Although... but..." really wrong?
In English it is. "Although" and "but" both signal contrast, so using both in one sentence is redundant and counts as a grammar error. Keep one of them: "Although prices rose, demand held" or "Prices rose, but demand held". The mistake is common among learners whose first language joins the two.
Is a long sentence better than a short one in IELTS?
No. The band 8 explorer cell warns against long structures used for display where a short sentence was the right choice. Length is not complexity. A short sentence that lands cleanly is worth more than a long one with a mistake inside it, and a mix of both is what the descriptors reward.
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